And then heard a small what sounded similar to a "sizzle' and saw a small light inside side of harddrive as I have a clear side panel.  It appeared to come from area of graphics card.  I rebooted, and all seems ok, I did lose all my cookies in Firefox however.  Nothing else appears to be wrong with it.  Any ideas as to the cause?  Very wierd.  Anyway,   thanks for any info.HDD's are sealed...thats not where the light came from...Your best bet would be to open the machine and look around for any sign/smell of burning - If something blew up enough to cause light it should have left a burn mark somewhere on a component.Computer powered off by itself...
is this happening again and again.

Many times this happens due to high temperature. Check your cooling fans, Heat SINKS, etc..Open up the Power Options Control Panel. If you're on a LAPTOP, this can easily be done by right clicking on the battery icon.

You now have two options. You can change one of the existing plans to "Never" shut down the DISPLAY or system, or create a new power plan - call it "Presentation Mode".

Check your current power plan by clicking the Change Plan Settings link. Change the settings accordingly. On a desktop, there's a single set of settings, on laptops there are two SETS - one for battery operation and the other for AC adapter running. MAKE sure you change both sets for Presentation.